As an undergraduate research assistant with the Center for Human Frontiers I worked as a part of the Project Lim[b]itless team, which was focused on creating 3D-printed lower-limb prosthetic sockets and utilizing photogrammetry and digital image correlation software to make prosthetic sockets more accessible and affordable for amputees in developing regions of the world.

As part of this team I was brought on to determine a way to reinforce these 3D-printed prosthetic sockets such that they would be compliant with ISO strength requirements, as the inherent anisotropy of a 3D-printed socket could lead to a flexural failure through layer delamination just from walking using them. To do so I investigated several different methods of strengthening fused deposition modeling (FDM) 3D-printed sockets, looking into ways of coating the resulting sockets with resin, annealing the sockets after being printed, and even utilizing different 3D-printed materials. Eventually, through a literature review I landed on this method of reinforcing 3D-prints with epoxy resins called fill compositing, which involves permeating resin into a porous 3D-print with fully-connected infill.

After gaining some familiarity with composite layups, I led the development of a novel method for reinforcing additively manufacturing prosthetic sockets via fill compositing by permeating resin into a prorous 3D-printed specimen with vacuum infiltration techniques. The top image shows the method I determined for this, which relies on a vacuum pump drawing a vacuum through a 3D-printed part with infill exposed on both ends. The part was wrapped in vacuum bagging and on one end of the part was attached a breather/bleeder cloth which would allow a vacuum to be drawn through it but minimize the amount of resin that leaks through the tubes, and on the other end was a resin jacket which caused the resin to move evenly through the 3D-printed part. This setup allowed for epoxy and urethane resins to be drawn through the part evenly, reinforcing the 3D-printed parts.

Utilizing this technique I designed testing specimens, in accordance with ASTM D638, which could be water-jet cut out of a sheet of 3D-printing material which was infiltrated with resin. These specimens were then tested on an Instron electromechanical testing system and the results were verified using the two-dimensional image correlation package Ncorr. Through these tests, it was determined that a combination of using CF PLA or CF PETG filament with infiltrated urethane resin reduced the anisotropy of the 3D-printed parts and improved flexural strength.

I then developed a 3D-model of a residual limb utilizing photogrammetry and Ncorr before designing a lower-limb prosthetic socket in Meshmixer and printing that socket out. Using the vacuum-infiltration fill compositing techniques, I permeated the socket with resin before cutting it in half to determine how completely it was permeated with resin. Unfortunately due to graduation and the pandemic I was unable to properly test the socket itself, but as can be seen on the image on the right the infiltration was nearly 100% which signified a successful end and a bright future for this research in the future. The publication of this research can be found here:

Cabrera, I.A.; Hill, P.J.; Zhao, W.-Y.; Pike, T.C.; Meyers, M.A.; Rao, R.R.; Lin, A.Y.M. Prosthetic Sockets: Tensile Behavior of Vacuum Infiltrated Fused Deposition Modeling Sandwich Structure Composites. Prosthesis 2022, 4, 317-337. https://doi.org/10.3390/prosthesis4030027 

Working on Project Lim[b]itless gave me a great understanding of 3D-printing, prototyping techniques, materials science, experimental design, team leadership, and technical writing.